Porous membranes, such as microfiltration membranes or ultrafiltration membranes, have been used in a wide range of fields for such purposes as treating industrial waste and other such sludge substances, and sterilizing pharmaceutical water. Membranes used for these purposes can be broadly divided into hydrophobic porous membranes and hydrophilic porous membranes, and in the field of solid-liquid separation, hydrophobic porous membranes are preferable because of their chemical resistance, fouling resistance, weather resistance, resistance to degradation by oxidation, and so forth. However, water or an aqueous solution cannot pass through the pores of a hydrophobic porous membrane because of the hydrophobic nature of the membrane, or considerable pressure must be applied even if such passage is possible. Therefore, hydrophobic porous membranes are subjected to a hydrophilization treatment ahead of time so that water or an aqueous solution can pass through more easily. This hydrophilization treatment is necessary not only when a hydrophobic porous membrane is to be used for the first time after its manufacture, but also whenever all or part of the membrane dries out through contact with air during the inspection or cleaning of the membrane or during an extended shut-down of operation. The appropriate hydrophilization of a hydrophobic porous membrane is particularly important with a membrane composed of a polymer that is highly hydrophobic, such as a fluororesin, because there is a pronounced decrease in liquid permeability once the membrane is dried.
Examples of known methods for hydrophilizing a hydrophobic porous membrane include a method in which hydrophilic groups are introduced into the membrane itself (Japanese Laid-Open Patent Application H6-296686), as well as treating the membrane with deaerated water (Japanese Laid-Open Patent Application H5-208121), alcohol (Japanese Laid-Open Patent Application S58-96633), glycerol (Japanese Laid-Open Patent Application 2002-95939), or an inorganic salt (Japanese Laid-Open Patent Application H6-277470).
However, with a method in which hydrophilic groups are introduced into the membrane itself (Japanese Laid-Open Patent Application H6-296686), the membrane has to be thoroughly washed with a cleaning liquid, such as a large amount of water, to remove any of the monomer constituting the hydrophilic groups that remains unreacted on the membrane. With a method in which the membrane is treated with deaerated water (Japanese Laid-Open Patent Application H5-208121), deaerated water essentially has to be pressurized and passed through the membrane, so this treatment method is complicated. Furthermore, since the hydrophilized membrane must be kept moist at all times, a module containing the hydrophilized membrane has to be transported, shipping, sold, and so forth in a state of being filled with moisturizing liquid or the like, which means that it handling is inconvenient. With a method in which the membrane is treated with alcohol or the like (Japanese Laid-Open Patent Applications S58-96633, etc.), the alcohol or other substance used for the treatment remains in the hydrophobic porous membrane, so the membrane has to be thoroughly washed with a large amount of cleaning liquid at the time of its use.
Also, a method in which a specific process is employed to treat a membrane with a surfactant has been disclosed as another hydrophilization treatment method for a hydrophobic porous membrane (Japanese Laid-Open Patent Application H1-119310).
Japanese Laid-Open Patent Application H1-119310, however, is employed the specific process to reduce the amount of surfactant used and thereby minimize a drawback of elution on the basis that there is the drawback in that when a membrane is treated with a surfactant, some of the surfactant remains behind and is gradually eluted into the treated water. In other words, Japanese Laid-Open Patent Application H1-119310 merely involves reducing the amount of surfactant used and thereby reducing the amount of surfactant that is eluted, and does not solve the fundamental problem of surfactant elution.